Newborn infants (21 preterm and 13 term) received dopamine infusions at a low (2.5-3.4 micrograms/kg per min) and/or high (5-10 micrograms/kg per min) infusion rate and changes in plasma catecholamines were monitored. The mean baseline values for dopamine, noradrenaline and adrenaline were between 240 and 560, 125 and 144 and 62 and 82 pg/ml, respectively. During low-rate infusion of dopamine, there was a significant increase in plasma dopamine (20-100 fold), noradrenaline (three- to five-fold) and adrenaline (threefold). Administration of dopamine at the high rate resulted in an even larger increase in the plasma catecholamines (dopamine, 100-300 fold; noradrenaline, seven- to eightfold; adrenaline, four- to sixfold). In a double-log plot, there was a highly significant correlation between the plasma concentrations of dopamine and noradrenaline (r = 0.77; P less than 0.001). In conclusion, infusion of dopamine in term and preterm newborn infants is accompanied by an enhanced sympatho-adrenal tone which may contribute to the cardiovascular effects of dopamine in these patients.